An automatic clutch of a vehicle such as an automobile often includes a torque converter which is disposed between an engine and an automatic transmission, and which has a lockup clutch. A patent document 1 discloses the control of the lockup clutch configured to prohibit a full engagement (lockup) of the lockup clutch when an accelerator is switched to an ON state at the lockup (that is, a coast lockup) during a coast traveling by an accelerator OFF, for avoiding a large torque shock generated by an operation of a reverse torque in which a torque transmission direction is reversed.
As described above, when the accelerator is switched to the ON state at the coast lockup and the full engagement of the lockup clutch is prohibited, the lockup clutch is brought to a full disengagement state or a slip engagement state so as to avoid the torque shock. Then, the control is performed to rapidly bring the lockup clutch to the full engagement state (drive lockup) again. This is effective for improving the fuel consumption.
When the accelerator is switched to the ON state in a state where the lockup clutch is disengaged or slip-engaged, the engine speed is increased, and the engine torque is increased and transmitted by the torque converter. Accordingly, a rotation speed of a turbine runner (turbine rotation speed) is also increased, so that the vehicle is accelerated or started. Then, the lockup clutch is fully engaged.
When the lockup clutch is disengagement state, the lockup clutch is firstly slip-engaged when the disengaged lockup clutch is engaged. With this, a rotation speed difference between input and output elements of the lockup clutch (a rotation speed difference between the engine speed and the turbine rotation speed) is decreased. Then, the lockup clutch is shifted to the full engagement state. Besides, in a process to the full engagement state, a clutch judder in which a forward and rearward G of the vehicle is vibrated (judder vibration) is often generated.
The judder vibration is caused by a variation of a difference between a static friction coefficient and a dynamic friction coefficient. In particular, in a case where the automatic transmission is the continuously variable transmission, when the continuously variable transmission is upshifted in accordance with the increase of the turbine rotation speed, the turbine rotation speed is suppressed. Accordingly, the engagement control decreases the engine rotation speed towards the turbine rotation speed. Therefore, the torque increase function of the torque converter is largely varied, so that the engine load is largely varied. The amplitude of the judder vibration becomes large.
Moreover, the causes of the generation of the judder vibration are the magnitude of the rotation speed difference, the relationship between the engine torque and the transmission torque capacity of the lockup clutch (corresponding to the engagement pressure of the lockup clutch). For example, when the engine torque is increased by an additional depression of the accelerator pedal at a timing at which the rotation speed difference becomes small immediately before the full engagement of the lockup clutch, the transmission torque capacity of the lockup clutch with respect to the engine torque is deficient. With this, it is not possible to attain the smooth shift to the full engagement to cause the judder vibration.
It is, therefore, an object to provide a control device for an automatic transmission to dissolved the above-described problems, and to avoid a judder vibration generated due to a torque increase of a driving source at a transition to a lockup state.